An engine without a prop doesn't make much thrust. A static (non turning) prop makes no thrust. A prop turned at xxx rpm will make the same thrust on any engine turning it. Each brand of prop will make a differing amount of thrust at the same rpm.
RC Report magazine publishes test reports using the same set of test props and on the same bench with the same fuel and even a correction factor for the weather. For years I've found their reports to unbiased and accurate. Some times they will even try some different brands of props and different fuel in addition to the standards for extra information. I can refer to a 5 year old report on an engine and compare it to a current one without fear of differing test conditions.
I appreciate the shootouts from MAN and RCM but they are only good for those engines included in that article as the test conditions and author are often different in the next test report.